SILACYCLOPROPYNE - MATRIX SPECTROSCOPIC IDENTIFICATION AND AB-INITIO INVESTIGATIONS

Matrix-isolated 1-silacyclopropenylidene (7) has been generated by pul sed flash pyrolysis of 2-ethynyl-1,1,1 -trimethyldisilane (4). By subs equent photolysis 7 can be isomerized into ethynylsilanediyl (5), viny lidenesilanediyl (8), and silacyclopropyne (9). The identification of the C2H2Si isomers and their C-13 and D isotopomers is based on the co mparison of their experimental and calculated (MP2/6-31G*) infrared s pectra. In case of 9, the first example of a ''formal'' cyclopropyne, the definitive assignment of the structure was only possible by studyi ng C-13 isotopomers. Detailed ab initio calculations have been carried out in order to understand the unique bonding situation in 9. It is f ound that the Lewis structure 9B, where the dashed line indicates an e lectron pair which occupies a CC nonbonding orbital, is the best possi ble compromise to describe the electronic structure of 9 correctly. Th e value of 6.2 kcal mol(-1) for the barrier of 9 to the planar transit ion structure 13 represents the lowest inversion barrier of tetra-coor dinated silicon described so far.